It's true that the relations between Muslims and their pagan fellow-tribesmen rapidly deteriorated. Professor Marco Scholler teaches that pagans persecuted Muhammad early on and that he "suffered from humiliation, derision and from being treated either like a madman or an outcast. Some people would even fling pebbles at him while he was praying and others kicked stones at him so that he had to run away with bleeding feet."
Christians were not doing this to Muhammad: pagan Meccans were.
Yes that is correct. The Romans and Persian were not interested in the Arab Peninsula. They referred to Arabs as people who eat the skin of dead animals and not worthy to think about. When the Muslims started to unite under one leader then that changed.